This invention relates to integrated circuits, and more particularly, to integrated circuits with metal-oxide-semiconductor transistors that have enhanced gate depletion layers for increasing the resistance of the transistors to voltage-induced degradation.
Metal-oxide-semiconductor (MOS) transistors are commonly used to form circuitry on integrated circuits. For example, integrated circuits often use complementary metal-oxide-semiconductor (CMOS) transistor technology. CMOS integrated circuits have n-channel metal-oxide-semiconductor (NMOS) and p-channel metal-oxide-semiconductor (PMOS) transistors.
NMOS and PMOS integrated circuits have four terminals—a drain, a source, a gate, and a body. The body, which is sometimes referred to as the well of a transistor, is typically formed from silicon. A doped body contact is used to form a body terminal. For example, re-channel transistors have bodies that are doped p-type. In a p-type body, the body contact is formed from a heavily doped p+ region. Source and drain terminals are formed by doping source and drain regions within the body. In an n-channel transistor, the source and drain regions are heavily doped with n-type dopant (i.e., the source and drain regions are doped n+).
In each transistor, a gate is formed between the source and drain. The gate includes a silicon oxide insulating layer. A gate conductor is formed on top of the gate insulator. The gate conductor may be, for example, a layer of metal. In modern integrated circuits, the gate conductor of an MOS transistor is typically formed from heavily doped polysilicon. The gate conductor in this type of transistor is generally doped as heavily as possible to ensure that the transistor switches rapidly during operation.
In many system environments, different transistors on an integrated circuit are exposed to different voltages. For example, the interior or core portion of many integrated circuits operates at a relatively low power supply voltage. Low voltage core logic can help to reduce power consumption. Low voltage designs can also be useful when forming transistors with relatively small dimensions (i.e., when scaling a design to a small size to increase circuit density).
Input-output circuitry on an integrated circuit is often located around the periphery of the integrated circuit. Input-output circuitry is used to provide an interface between the core logic of the integrated circuit and external circuitry. For example, input-output circuitry can be used to send and receive signals over parallel and serial data paths. To provide sufficient signal strength to overcome noise, the voltages of the data signals that are transmitted over the parallel and serial data paths may be larger than the voltages of the signals within the core logic of the integrated circuit. For example, the signals on external data paths may have voltages of three volts or more, whereas core logic may operate at signals of less than one volt.
The transistors in the input-output circuitry or other circuitry on an integrated circuit that handles elevated voltages must be more robust than the transistors in the core logic. For example, it may be desirable to form thicker gate oxides in the MOS transistors in input-output circuits than in the MOS transistors of core logic circuits. By forming thicker gate oxides in the input-output transistors, the input-output transistors may be made more resistant to breakdown induced by hot carriers.
It is generally not advantageous to form MOS transistors with thicker gate oxides than is necessary to resist hot carrier breakdown. This is because transistors with overly thick gate oxides tend to switch slowly. If, for example, all of the transistors in the core logic of an integrated circuit were fabricated with gate oxides sufficient to withstand high voltage input-output data signals, the switching speed of the core logic would be significantly reduced. Such reduced switching speeds are often unacceptable.
As a result, some integrated circuits use different transistor designs for different parts of the circuit. Input-output transistors that are exposed to a relatively high maximum voltage are provided with the thickest gate oxide layers. Core logic transistors that are exposed to a relatively low maximum voltage are provided with the thinnest gate oxides layers to ensure adequate switching performance. Still other transistors, which are exposed to voltages of intermediate magnitude, may be provided with gate oxides having a thickness that lies between the core logic gate oxide thickness and the input-output circuitry gate oxide thickness.
As circuits become more complex, it may become increasingly desirable to provide a range of input-output interface options. For example, it may be desirable for the input-output circuitry on an integrated circuit to support communications protocols that have different associated voltage levels. When supporting a range of protocols, it may be necessary to construct input-output transistors to handle the worst case voltage levels that are expected to be encountered. As an example, it may be necessary to form all input-output circuit transistors in an input-output circuit with a relatively thick gate oxide to ensure that these transistors will not be damaged by hot carriers in the event that relatively high voltage signals are encountered. However, providing all input-output transistors with relatively thick gate oxides will reduce transistor switching speeds below what is necessary in the event that only lower voltage input-output signals are encountered.
An alternative to enhancing the voltage handling capabilities of all input-output circuit transistors by providing them with thick gate oxides is to enhance the voltage handling capabilities of a limited number of the input-output circuit transistors. With this type of approach, some of the input-output transistors may be provided with thick gate oxides to handle the relatively larger voltages that are associated with a legacy communications protocol, whereas other input-output transistors may be provided with somewhat thinner gate oxides to handle lower input-output voltage levels. Core logic transistors may be fabricated with an even thinner gate oxide thickness to ensure optimum switching speeds are obtained for the main processing circuitry on the integrated circuit.
Although this type of arrangement may be satisfactory in some circumstances, there can be significant disadvantages to arrangements with large numbers of different transistor gate oxide thicknesses. Each transistor gate oxide thickness that is supported increases the complexity of the semiconductor fabrication process that is used during manufacturing. If the process that is used to manufacture an integrated circuit with multiple transistor gate oxide thicknesses becomes too complex, manufacturing yield and throughput may degrade to unacceptable levels.
It may therefore be impractical to provide integrated circuits with as many different transistor types as desired. As a result, certain transistors may have oxides that are too thick for optimum performance or signals with certain voltage levels may not be supported by the integrated circuit. Device performance and compatibility with legacy protocols may therefore suffer.
In view of these challenges it would be desirable to be able to provide improved integrated circuits with transistors capable of supporting different voltage levels.